Calcium ion channels are membrane-spanning, multi-subunit proteins that allow Ca2+ entry from the external milieu and concurrent depolarization of the cell's membrane potential, and play a central role in neurotransmitter release. Traditionally calcium ion channels have been classified based on their functional characteristics such as low voltage or high voltage activated and their kinetics (L, T, N, P, Q). The ability to clone and express the calcium ion channel subunits has lead to an increased understanding of the channel composition that produces these functional responses. Calcium channels can be classified into a number of types and subtypes, for example L-(or Cav1), P/Q-(or Cav2.1), N-(or Cav2.2), R-(Cav2.3) and T-(or Cav3) types. T-type calcium channels can, for example, be molecularly, pharmacologically and electrophysiologically sub-classified into α1G (or Cav3.1), α1H (or Cav3.2), and α1I (or Cav3.3) T channels from various warm blooded animals including rat. The “T-type” (or “low voltage-activated”) calcium channels are so named because their openings are of briefer duration (T=transition) than the longer (L=long-lasting) openings of the L-type calcium channels. The L, N, P and Q-type channels activate at more positive potentials (high voltage activated) and display diverse kinetics and voltage-dependent properties. See e.g. Catterall, Annu. Rev. Cell Dev. Biol. 16, 521-55, (2000) and Perez-Reyes Physiol. Rev. 83, 117-161, (2003).
The pharmacology of the three subfamilies of calcium ion channels is quite distinct from each other. The type I Cav1 (L-type) channels are distributed within cardiac muscle, smooth muscle including blood vessels, intestine, lung, uterus, skeletal muscale, endocrine cells, and are the molecular targets of the organic calcium channel blockers used widely in the therapy of cardiovascular diseases.
The type II Cav2 (P, Q, N. R) channels are in neurons, heart, etc. They are relatively insensitive to dihydropyridine calcium channel blockers, but these channels are specifically blocked with high affinity by peptide toxins from spiders and marine snails. The N type Ca2+ channel (Cav2.2) is highly expressed at the presynaptic nerve terminals of the dorsal root ganglion as it forms a synapse with the dorsal horn neurons in lamina I and II. These neurons in turn have large numbers of N type Ca2+ channels at their presynaptic terminals as they synapse onto second and third order neurons. This pathway is very important in relaying pain information to the brain. The N type Ca2+ channel has been validated in man by intrathecal infusion of the toxin Ziconotide for the treatment of intractable pain, cancer pain, opioid resistant pain, and neuropathic and severe pain. The toxin has over 80% success rate for the treatment of pain in humans with a greater potency than morphine. However, Ziconotide causes mast cell degranulation and produces dose-dependent central side effects. These include dizziness, nystagmus, agitation, and dysmetria. There is also orthostatic hypotension in some patients at high doses. It is believed that this may be due to Ziconotide induced mast cell degranulation and/or its effects on the sympathetic ganglion that like the dorsal root ganglion also expresses the N type Ca2+ channel. Use-dependent compounds that block preferentially in the higher frequency range >10 Hz should be helpful in minimizing these potential side-effect issues. The firing rate in man of the sympathetic efferents is in the 0.3 Hz range. CNS neurons can fire at high frequencies but generally only do so in short bursts of action potentials. Even with the selectivity imparted by use-dependence intrinsic selectivity against the L type calcium channel is still necessary as it is involved in cardiac and vascular smooth muscle contraction.
The third type Cav3 (T-type) channels exist in brain, heart, kidney, liver, etc. They are insensitive to both the dihydropyridines that block Cav1 channels and the spider and cone snail toxins that block the Cav2 channels. T-type Ca2+ channels are new anticipated to be novel therapeutic targets for the treatment of various cardiovascular disorders such as heart failure, arrhythmia, hypertension, neuronal disorders such as epilepsy and pain, as well as cancer. Inhibition of T-type Ca2+ channels may result in long-term organ protection through improvement of local microcirculation and reduction of adverse hormonal effects. However, there are no widely useful pharmacological agents that block T-type calcium currents. The organic calcium channel blocker mibefradil is somewhat selective for T-type versus L-type calcium current and showed strong side effects due to drug interaction at the cytochrome P-450 3A4 enzyme which was unrelated to T-type Ca2+ channel blockade. The peptide kurtoxin inhibits the activation gating of Cav3.1 and Cav3.2 channels. Development of more specific and high-affinity blockers of the Cav3 family of calcium channels would be useful for therapy and a more detailed analysis of the physiological roles of these channels. The T-type Ca2+ channel has properties different from those of the L-type such as more negative voltage range of activation and inactivation, rapid gating kinetics, and resistance to standard Ca2+ blockers such as Ca2+ channel blockers, which block L-type Ca2+ channels.
T-type calcium channels have been implicated in pathologies related to various diseases and disorders, including epilepsy, essential tremor, pain, neuropathic pain, schizophrenia, Parkinson's disease, depression, anxiety, sleep disorders, sleep disturbances, psychosis, schizophreniac, cardiac arrhythmia, hypertension, certain types of cancer, diabetes, infertility, sexual dysfunction and cancer (J Neuroscience, 14, 5485 (1994); Drug Future 30(6), 573-580 (2005); EMBO J, 24, 315-324 (2005)).
The known therapeutic regimens for such treating such diseases and disorders suffer from numerous problems and a number of side effects. These side effects include various CNS disturbances such as blurred vision, dizziness, nausea, and sedation as well more potentially life threatening cardiac arrhythmias and cardiac failure. Accordingly, a more physiological way, to develop additional Ca2+ channel blockers/antagonists, preferably those with higher potency, high selectivity and fewer side effects, to treat these diseases and disorders would be highly desirable.